1. Field of the Invention
This invention relates to variable delay equalizers for compensating for delay distortions caused in the transmission of signals over FM and PCM digital communications systems, and more particularly to variable delay equalizers of the type using a directional coupler and a variable phase shifter.
2. Description of the Prior Art
One typical prior art delay equalizer consists essentially of a directional coupler having first and second input terminals and first and second output terminals, the second output terminal being connected to the second input terminal by way of a feedback path. In this type of equalizer, the input frequency vs. delay characteristic has a peak delay at a wavelength (frequency) relevant to the length of the feedback path, and the delay tends to decrease as the frequency departs from the center of the characteristic frequency of the device. Hence, by changing the length of the feedback path, the delay of an input signal within its transmission band can be equalized. Although this approach enables the peak delay to be shifted with respect to frequency, it is nearly impossible to change the characteristic delay value itself, i.e., the peak delay time.